Hi there,
I've done this using the breakout module from Spark Fun. Basically, assuming the atmega8, you connect analog 4 to SDA and A5 to SCL, and attach a pull-up resistor (I think I used 2.2k) from both of these to the 5V line. I used the data sheet for the chip to figure out how to program it and read it over I2C. Here's my code for a circuit that reads the time and displays the hours/minutes/seconds on a single 7 segment display. I use straight C++ outside the IDE, so you can probably kill the #includes.
/*
* A single-digit clock
* (c) 2007 Bob Copeland <me at bobcopeland.com>
*/
#include <WProgram.h>
#include <Wire.h>
#include <HardwareSerial.h>
char bcd_to_7seg[] = {
0x3f, 0x06, 0x5b, 0x4f, 0x66, 0x6d, 0x7d, 0x07, 0x7f, 0x6f, 0x00
};
// pins a-g on 7 seg display
int pins[] = { 11, 10, 7, 3, 4, 8, 12 };
void setup()
{
int i;
Wire.begin();
Serial.begin(9600);
for (i=0; i<7; i++)
pinMode(pins[i], OUTPUT);
/*
// program the time & enable clock
Wire.beginTransmission(0x68);
Wire.send(0);
Wire.send(0x00);
Wire.send(0x52);
Wire.send(0x80 | 0x21);
Wire.endTransmission();
*/
}
void write_digit(char digit)
{
int i;
char byte = bcd_to_7seg[digit];
for (i=0; i<7; i++)
{
digitalWrite(pins[i], byte & 1);
byte >>= 1;
}
}
void write_number(char num)
{
write_digit(10);
delay(50);
write_digit((num >> 4) & 0x0f);
delay(200);
write_digit(10);
delay(50);
write_digit(num & 0x0f);
delay(200);
}
void loop()
{
// reset register pointer
Wire.beginTransmission(0x68);
Wire.send(0);
Wire.endTransmission();
delay(100);
Wire.requestFrom(0x68, 3);
char secs = Wire.receive();
char mins = Wire.receive();
char hrs = Wire.receive();
write_number(hrs & 0x3f);
delay(100);
write_number(mins);
delay(100);
write_number(secs & 0x7f);
delay(1000);
}
